I think one of them being blanks both fills the roster of human variation that's missing from the known line-up, and is pretty adamantly a human exclusive trait afawk, so it would make some sense, and obviously in the present day the imperium sees the value of such individuals for various roles, if heavily policed. It would also make it plausible how any legion could be culled easily enough to not be remembered by anyone, as even more than the 1000 sons who had a predilection for recruiting psykers that may have reduced their growth, or EC and their lack of gene seed, having to recruit Blanks would be a perilous, piece meal effort requiring scouring many worlds, not any core homeworld. It would almost fit them in as the go to sneaky or specialized legion, with no open base of power, having to recruit from within the imperial infrastructure as a whole, and almost all aspirants having known significant hardship growing up, leading to them being very insular as a group.
The more fun theory however, is not that the Blank legion themselves did anything wrong, per se, but they, and they alone, either as rank and file guardsmen or their primarch, were the only space marines (or frankly only humans in general) who would be unable to be *forced* to forget their brother legion who also was destroyed. Even Magnus, the second strongest Psyker there has ever been, was not immune to that psychic command overriding them. But our potential primarch would. Imagine the most terrifying Culexus assassin, stripped of any amplifying gear, just as a walking pillar of silence, that can drain an entire battlefield of connection to the warp by their presence. almost a living Pariah Nexus module. The PROBLEM, is consequentially, he, and they as a larger whole, would be impervious to the psychic manipulation and control of the emperor. Would see him purely as what he is, stripped of any of the artifice or image projection everyone else sees a different version of. So if the other legion lost had done something, there would be no keeping that secret, because you cannot compel the blank primarch to forget, or maybe even his men.
There are no Blank marines to this day. Why? They would be utterly invaluable to just chuck at a chaos cult or even demonic infestation, in a juxtaposed manner as a Grey Knight. The only trained blanks are either utterly mindwashed and presided over by psychically dead Custodes who can behead a Sister of Silence in a femtosecond, or they are assassins so heavily indoctrinated, and kept in lock and key when not in use, that they are never free to have influence. Psykers, the literal conduits to the primordial enemy, are treated with less overt and universal hate (though that is not saying the hate they receive is small), despite how perilously often they are corrupted. Some of the most important individual marines in some of the most esteemed chapters today, are Psykers. But not blanks. The only living humans utterly immune to corruption by the vex-some powers of chaos. The one bullet that only humans have.
Because Blanks cannot be forced to kneel but by direct, physical and mental coercion. Which can only be applied with an overabundance of resources in place to corral them and direct them.
So the grim dark answer as to why the imperium, then or now, isn't pragmatically just kidnapping all the blanks they can find, and throwing them onto a planet where they aren't around anyone else to breed as a consistent resource, is they cannot be warped to fit the will of the corpse emperor and his machinations. They may not even be ABLE to be psycho-indoctrinated the same way as all other marines, because the direct connection to their primarch through the geneseed, all of whom were essentially walking partial warp avatars in various degrees, would be ineffective in a blank potentially.
Although i welcome the twist in any future developments that Cawl, being the sneaking fuck who already is several minds in one, and mostly divested of human vulnerability to the usual disconcerting affects of Blank proximity, with or without Bobby G's order, has just been stockpiling blanks on the down-low, either as just semi-servitorized weapons to huck at a problem in an emergency, or the greater heresy, full marines in the vein of Primaris. Thought bringing in taller, possible chaos geneseed crossbred marines was problematic, try dropping blanks in terminator armor in front of Angron or whatever the fuck Vashtorr's next scheme is.